The present invention relates generally to implantable medical devices, and more particularly to an implantable cardioverter/defibrillator device which provides improved efficiency and energy conservation by delivering a defibrillating shock waveform at the optimum time to a patient's heart in fibrillation. The invention further relates to techniques for more reliable and reproducible determination of the minimum energy requirements for defibrillation.
The importance of speed and effectiveness in reviving/resuscitating/defibrillating a patient whose heart is in fibrillation cannot be overemphasized. Implantable cardioverters/defibrillators have the capability to automatically detect abnormal heart rhythm or rate as it occurs and to respond with virtually immediate delivery of an appropriate preselected therapy. Detection of abnormal tachycardia or of fibrillation by the device is initiates automatic delivery of an electrical shock waveform (or, as sometimes termed in this specification, simply "shock") to cardiovert or defibrillate the patient's heart.
Because such devices are battery operated, it is, of course, essential that energy be conserved so that a sufficient amount is available for defibrillation or other available therapy each time it is required by the patient as detected by the device. The condition of the device, including its battery level, is checked periodically by the patient's cardiologist during scheduled visits, but if undesired factors are present it is possible that the battery may nevertheless become depleted or fall to a level insufficient to meet demand for therapy during a cardiac event that occurs before the problem can be discovered at the doctor's office. For a patient whose heart is prone to fibrillation, such a circumstance can prove to be fatal. At the very least, these circumstances will necessitate surgery at more frequent intervals for device replacement.
To conserve energy in the implantable cardioverter/defibrillator, improved techniques have been developed to avoid device operations that are wasteful of energy, such as false shocking, i.e., delivery of a shock in response to a false detection of tachycardia or fibrillation, or committed operation, in which, once capacitor charging is commenced, a shock will be delivered despite that fibrillation may have spontaneously ceased in the interim. The improvements in energy conservation include better discrimination of cardiac events which require shocking the heart from those events which do not suck treatment. To that end, devices have been implemented with improved detection criteria, and with uncommitted high energy shock therapy so that if, for example, a tachycardia or fibrillation terminates spontaneously, the otherwise inexorable movement toward delivery of a shock is interrupted at that point despite the fact that charging of the device capacitors may have commenced.
Despite such advances, it is essential that when a shock is determined to be appropriate to the patient's cardiac condition, it is delivered at a time when it is most likely to successfully terminate the dysrhythmia and return the heart to normal sinus rhythm. Further, the optimum time should coincide with a point of greatest likelihood that the minimum energy required is delivered to achieve that result, i.e., that the shock is applied when the defibrillation threshold is likely to be lowest. Without such timing, the device may still be depleted of energy prematurely as a result of either the need for delivery of multiple shocks to terminate the episode, or of a single shock which is of higher energy than absolutely necessary because it is applied at a time of high DFT.
U.S. Pat. No. (USPN) 4,384,585 to Zipes describes synchronous cardioversion with the possibility of inducing fibrillation by non-synchronous shocking. In that patent disclosure, it is stated that synchronous cardioversion delivers the shock at a time when the bulk of the cardiac tissue is already depolarized and is in a refractory state, whereas non-synchronous cardioversion is avoided to preclude delivery of cardioverting energy during the vulnerable T-wave portion of the cardiac cycle. Electrogram information from the patient's heart is used to detect depolarizations of the cardiac tissue and to produce a corresponding sense signal. Detection criteria are applied with respect to the sense signal to determine whether a tachyrhythmia is present.
In the '585 patent device as described, the detection circuitry determines the time interval between successive cardiac depolarizations, and initiates a discharge of energy stored in an output storage capacitor if either the average detected heart rate is above a preset threshold for a limited period of time or if the detected heart rate accelerates by a preset amount. Alternatively, the criteria imposed involve detection of a departure of selected beats from a historic data base of a succession of R--R intervals stored in memory; or a waveform analysis of the electrogram information with pattern recognition of time domain or frequency domain characteristics of the tachyrhythmia signal. The shock is delivered in synchrony with the R-wave to avoid a manifest ventricular response.
In U.S. Pat. No. 4,996,984 to Sweeney, a defibrillation method is described in which fibrillation cycle length--constituting the average time interval between successive depolarizations--is measured and stimulation is applied to the heart in the form of multiple bursts of electrical current timed according to that cycle length. As that patent specification points out, when cells of cardiac tissue are activated, the normal electrical polarization represented by the voltage difference within and outside the cell collapses, or depolarizes. Repolarization of the cell then commences, in which this voltage difference is reestablished. Before the repolarization process is completed the tissue is refractory, and afterward it becomes non-refractory--the period from depolarization to non-refractoriness being termed the refractory period. The depolarization activity propagates through the heart, as cells are activated by the collapsing polarization (i.e., depolarization) of adjacent cells.
The '984 patent disclosure teaches that the multiple burst defibrillation technique is optimized by timing the successive bursts to occur at successive depolarizations at a particular site in the myocardium. Thus, the time interval between bursts is adjusted to correspond to the cycle length, to the extent it is feasible to do so. Fibrillation cycle length is determined, according to the patent, by methods such as cross-correlation, auto-correlation, fast Fourier transformation, counting the R-waves of the electrocardiogram over a fixed time period and determining the R--R intervals of individual cardiograms.
In U.S. Pat. No. 5,275,621 to Mehra, a cardioverter device is disclosed which employs a pair of electrodes, one for measuring a near field and the other for measuring a far field, and for developing corresponding near field and far field signals. A cardioversion pulse is delivered in synchrony with detection of the near field signal following detection of onset of the far field signal, using a pair of time intervals measured from onset of the far field signal. Despite a lengthy specification, the '621 patent appears to provide little insight into the significance of these fields, particularly as applied to a scheme for cardioversion.
It has been found that delivery of a cardioverting or defibrillating shock or other stimulating waveform according to teachings presented in the prior art does not result in the optimum time for delivery or in high reproducibility of success. All to often, the prior art proposals lead to triggering the shock at a time of relatively high defibrillation threshold (DFT). This presents a need for large devices, along with a greater likelihood that considerable energy may be wasted each time a shock is delivered, resulting in acceleration of battery depletion, and most significant, a lower likelihood of success in terminating the fibrillation.
It is a principal object of the present invention to provide an improved implantable cardioverter/defibrillator in which detection is such that it enables optimum timing of delivery of the prescribed electrical therapy to a patient's heart, with high reproducibility of performance.
Another object of the invention is to provide a method for reducing the level of energy required to be delivered from an implanted cardioverter/defibrillator to terminate a detected abnormal cardiac event, by optimum timing of the delivery.
A major aspect of the invention is the decrease in defibrillation energy requirements which allows the use of small and lightweight defibrillators.